I've found there's not much difference in the dorsal, here's one of Fantasy where you can see how crisp it is on her.
![storm10.jpg storm10.jpg](https://cdn.imagearchive.com/miniaturehorsetalk/data/attachments/10/10170-8e3317ae87dcf52eb1c9f374b0737a8c.jpg)
This is where pedigree is helpful, I know there is no dun behind her so it's just countershading. But due to how many non duns have dorsal stripes I never trust it on it's own as a dun mark.
Now she COULD be dun, but with sooty playing on her and no known pedigree it's very hard to be certain one way or the other.
![storm10.jpg storm10.jpg](https://cdn.imagearchive.com/miniaturehorsetalk/data/attachments/10/10170-8e3317ae87dcf52eb1c9f374b0737a8c.jpg)
This is where pedigree is helpful, I know there is no dun behind her so it's just countershading. But due to how many non duns have dorsal stripes I never trust it on it's own as a dun mark.
Now she COULD be dun, but with sooty playing on her and no known pedigree it's very hard to be certain one way or the other.